[Terry Monk]

GQ has it right for most folks IMO.
I tend to jump all over the place myself. Weird I know. For instance, I take my last weight that I was able to do for at least 3-4 reps. I don't figure a percentage or a certain amount of weight. I have a weight in my head and I shoot for it. It's basically a with-in reach goal, but something I really have to struggle for. I get myself all pysched up and go for it. If I get it, great. If not I knock it down to about 10lbs heavier than my last high weight and do it. At that point that weight feels "light". I know, not very conventional but it works for me.
I've also found that if I pyramid up, that I'm worn out by the time I get ready to max and I generally don't hit it. I've started doing a warm up set and then my first working set is my try at a personal max; while I'm still fresh.

I don't pretend that this method will work for anybody besides me. So do as others say and not as I do. :p